Human exposure to arsenic is mainly from dietary sources; low-dose chronic intake affects human health and may cause cancers in all organs. The International Agency for Research on Cancer identified arsenic as Group 1 human carcinogen. Rice is the top energy source (20%) for human and dietary staple for half of world population. However, in comparison to other terrestrial crops, rice accumulates much higher arsenic, a notorious environmental contaminant, due to anaerobic growing conditions. Among arsenic species, inorganic arsenic (iAs) is far more toxic than its organic counterparts.
The Food and Agriculture Organization/World Health Organization determined iAs lower limit on the benchmark dose for a 0.5% increased incidence of lung cancer (BMDL0.5) to be 3.0 μg/kgbw·d. Currently, China set iAs maximum level in rice at 200 ng g−1; the Codex Alimentarius Committee on contaminants in food proposed 200 and 300 ng g−1 draft iAs MLs in polished and raw rice, respectively. To uphold regulations and protect consumers, methods capable of iAs detection at ng g−1 level are much needed. Because rice is such an important crop, it was selected as the model matrix in this document.
Hydride generation (HG) separates toxicologically relevant arsenic species (TRS) from interfering matrix components using a gas/liquid separator. As a result, both sensitivity and specificity are dramatically enhanced, leading to extensive application to atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma (ICP)-optical emission spectrometry (OES), and ICP-mass spectrometry (MS).
Speciation can be carried out either prior to HG, or post HG. In the prior-to-HG stage, successful speciation schemes include solid phase extraction (SPE) and dispersive liquid-liquid microextraction (DLLME). Alternatively, HG of TRS of arsenic: AsIII, AsV, monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA), can be carried out under four sets of conditions (including variations in pH, reductant variety, and concentration). In this scenario, four linear equations are set up to correlate TRS concentrations to AFS signals. After all coefficients are obtained from standards of known concentrations, concentrations of TRS of arsenic in an unknown sample can be solved mathematically.
In the post-HG stage, cryogenic trapping (CT) and cryogenic focusing (CF) are effective separation techniques. These techniques are based on the boiling point (BP) of organic and inorganic arsines. For example, BP of resulting arsine species are as follows: AsH3 at −55° C., CH3AsH2 at −2° C., and (CH3)2AsH at 35.6° C., respectively.
FIG. 1 shows a bench-scale example of the prior art process used to separate various arsines. As shown in FIG. 1, the arsines of TRS are trapped in a U-tube immersed in liquid nitrogen (LN2). The U-tube is then heated by a coil (not shown) wrapped around the tube exterior, or the U-tube is simply heated by exposure to the ambient air. Rising temperature causes trapped arsines to release from the U-tube in the order from low to high BPs. The arsines are then swept by an argon stream to AFS or AAS detector. This method accomplishes TRS speciation without using chemical reagents and thus has cost advantages. Currently, liquid nitrogen is the most common coolant used in this process.
Although the prior art method is generally effective, there are multiple challenges/issues associated with the use of liquid nitrogen. If the liquid nitrogen used in the cooling module is handled improperly, it can cause damage to lab equipment and injury to lab personnel. The need exists for a safer and more reliable means of cooling and condensing the arsines of the TRS. As shown in the FIG. 2 schematic, the system described herein comprises a thermoelectric means of heating and cooling hydride gas that is a safer and more accurate than the prior art process. In the process described herein, thermoelectric Peltier modules cool the hydride gas as it passes through the cryotrap body so that no liquid nitrogen is required.